[Yaesu] Re: FT-1000D 4th IF / Notch and Prod Det - Frequencies? Function?

[Chris Codella, W2PA]

Jeff,

I took another look at the notch circuit today and have the following 
modification to my original explanation last weekend:

D2056 does indeed do the switching - and it's bypassing a conventional 
T-notch circuit, similar to the one used in the Drake R-4 series 
receivers (all of them).  But in those receivers, the inductor was tuned 
whereas in this one it's fixed and the everything else is shifted up and 
down as noted earlier.

One further note about notch performance.  In this kind of notch 
circuit, the resistance to ground between the two capacitors is a 
critical value that depends on the load resistance and the center notch 
tuned circuit.  That's why there is an internal variable resistor 
(VR2003) to get it aligned (there's one in the Drake too).  If you are 
noticing poor notch performance perhaps this resistor needs tweaking. 

By the way, the correct value of that resistor depends on both L and C 
in the T-notch.  Thus if you tune the notch using L, as is done in the 
Drake, you should really also adjust the resistor to be optimal (but 
it's fixed in both rigs).  Thus it seems to me that the FT-1000D notch 
should theoretically perform better than the Drake notch since in the 
Yaesu the resistor can remain fixed and still be optimally set.

73,
Chris, W2PA


KA9S - Jeff wrote:

>I would like to understand the frequency scheme of the 4th mixer, 4th 
>IF / notch filter, and product detector of my FT-1000D.
>
...

>(and it is - a crappy 
>notch compared to my Drake R-4C - why bother?). 
>
>
>Am I even close with this explanation of the 4th IF / notch and product 
>detector? Can anyone explain how this is actually working? Is there any 
>way to make the notch filter narrower / deeper? 
>
>
>Thanks - Expert insights always welcome. 
>
>
>73s - Jeff - KA9S
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